1. Field of the Invention
The present invention relates to a process for the manufacture of potassium salts and particularly, to a process for the preparation of potassium salts from the spent liquor made in the pulping of lignocellulosic materials.
2. Description of the Prior Art
The major alkaline pulping processes for the manufacture of chemical pulps are sodium-based processes. In larger pulp mills, spent pulping chemicals are recovered and regenerated through well known means. In a typical process sequence, spent pulping liquor is collected, concentrated and burned in a smelter-type furnace under reducing conditions. The resulting solid combustion products composed of sodium sulphide and sodium carbonate are dissolved in water for subsequent conversion to suitable sulphite cooking liquors. See, for example, U.S. Pat. Nos. 3,098,710 (Ahlborg and Cederquist), 3,508,863 (Kiminki and Keskinen), and 4,049,787 (Romantschuk and Vuojolainen). In another common approach, the concentrated spent pulping liquor is burned in a fluid-bed reactor under oxidative conditions. See, for example, U.S. Pat. Nos. 3,674,630 (Copeland) and 3,927,174 (Copeland). The end products of fluid-bed combustion are sodium sulphate and sodium carbonate. Because the product is neither pure sodium sulphate nor pure sodium carbonate, the market for this product mixture is very uncertain. For example, the glass industry would prefer to purchase a product that contains mostly sodium carbonate. In contrast, the detergent industry would desire a product that contains mostly sodium sulphate. Moreover, there are numerous low-cost sources of pure sodium sulphate and sodium carbonate. Furthermore it is well known from the phase chemistry of sodium sulphate and carbonate that a commercial scale separation is not practicable. Under these technical and market constraints, the resulting economic return for the pulp mill practising such technology of spent liquor disposal is often not satisfactory.
Casciani et al. (U.S. Pat. No. 3,020,195) have disclosed a process by which excess potassium sulphate is added to a recovery furnace burning waste liquor produced in a potassium-based kraft or sulphite pulping process. The object was to co-produce potassium carbonate for external sales, using the pulp mill's chemical recovery facilities. Casciani teaches the addition of potassium sulphate to a recovery furnace burning kraft (or sulphite) waste pulping liquor. Under reducing conditions, potassium sulphide and potassium carbonate are produced. Upon subsequent causticization, a kraft cooking liquor consisting of potassium hydroxide and potassium sulphide is made. In the auxiliary furnace operating under oxidizing conditions, potassium sulphate and potassium carbonate are made. Potassium sulphate would then be preferentially separated from the mother liquor for use as feed to the main reducing furnace. The potassium carbonate remaining in the mother liquor would be used for external purposes.
Materials balance considerations suggest that this scheme is impractical. With the selective removal of potassium as potassium carbonate, the excess sulphur in the potassium sulphate input will accumulate preferentially in the mill's pulping liquor system. The tolerable limit of sulphidity in the pulping liquor will be exceeded.
The amount of potassium carbonate that can be recovered for external usage will be in direct proportion to the amount of potassium sulphate needed to make up for the process losses of potassium and sulphur. If potassium is removed selectively, in whole or in part, as potassium carbonate, then the disposal of the residual sulphur entering the mill chemical circuit could become a very serious problem. And if the excess sulphur input can not be removed separately, e.g., via the flue gas, the sulphur content of the cooking liquor will increase rapidly. The sulphidity of kraft pulping liquor has strict limits for efficient pulping operations.
With increasingly stringent control of gaseous sulphur emissions in recent years, the requirement for sulphur-based chemical makeup (i.e., potassium sulphate in the present case) has declined considerably. The potassium/sulphur loss ratio is no longer in the same ratio as in potassium sulphate. In fact, kraft mills are now using hydroxide or carbonate type makeup chemicals to replace the loss of the pulping cation: sodium or potassium as the case may be. The required (smaller) make-up of sulphur is being supplied, in some cases, in the form of elemental sulphur. In the Casciani concept, the production of extra potassium carbonate would not be feasible, if potassium sulphate could not be used by kraft mills as make-up chemical.
Furthermore for the small pulp mills having production capacity of 300 or less tonnes of pulp daily, the economics of all above-mentioned chemical recovery methods are even less attractive. In particular, for sulphite pulp mills producing higher yield pulps, the spent pulping liquor with a low ratio of organic to inorganic substances is not technically amenable to burning without the use of supplementary fuels.
And yet, ex-plant effluent treatment of the said spent pulping liquor is an expensive alternative without any economic returns. Moreover, the sale of spent pulping liquor in the open market faces severe competition in terms of price and volume.
The use of potassium-base for the chemical pulping of lignocellulosic material has been known for many years to those skilled in the art. Yamada et al., Japan Patent Kokai No. 86,808/78, discloses the use of potassium hydroxide and molecular oxygen for the delignification of non-woody plant material such as straw and esparto grass. Friese, German Pat. No. 2,365,480, teaches a two-stage process using potassium hydroxide and sulphuric acid to obtain lignin and pulp from hardwoods. Commercialization of these and other approaches has not been realized because of the significantly higher prices of potassium hydroxide and potassium carbonate in comparison to sodium hydroxide and sodium carbonate respectively, as chemical makeup. Gilbert et al., U.S. Pat. No. 4,249,990, discloses a method for the removal of wood-originated potassium compounds from a sodium-base kraft mill pulping liquor system. The potassium compounds interfere with the operation of the closed-cycle kraft mill described by Rapson in U.S. Pat. No. 3,698,995. Gilbert et al., Canadian Pat. No. 1,113,207, teaches the substitution of sodium base with a potassium base in the kraft pulping process, to eliminate the needs for kraft mills to remove extraneous potassium compounds from its cooking liquor system. Build-up of potassium compounds in a sodium base kraft pulp mill arises from the relatively high concentration of potassium in the natural wood.